The fermentation temperature gradient - Myth?

[hopvine]

Most of us know that fermentation is an exothermic process, meaning it produces heat. That is why we often notice that the temperature reading on the carboy (if you have a Fermometer attached) is higher than ambient temperatures. A 6 - 8 degree difference between AIR temperature and WORT temperature (depending on the rigor of fermentation) is pretty common.

However, more and more often I'm seeing posts that state that the temperature of the wort at the CENTER of the carboy differs from the temperature of the wort near the outside, and therefor the temperature listed by a Fermometer strip or a thermometer attached directly to the carboy is drastically unreliable. I believe this is a misconception stemming from the fact that in a SOLID (i.e. a block of metal with a heat source at the center), there is very often a thermal gradient between the center and the outside. This science does not hold true for liquids with fluid currents.

Here's what we can say without a doubt:
1) Liquid is a better thermal conductor than air
2) Glass is a better thermal conductor than air
3) Fermentation is an active process (i.e. fermenting wort is not stagnant, it moves)

Taking those three facts into account, we can then deduce that:
1) The temperature of the carboy is influenced much more by the wort temperature than the air, because fluid is more thermally conductive
2) No drastic thermal gradient from edge to center is possible due to the high fluid currents in the fermenter, which establishes a constant thermal equilibrium.

I am not claiming that a fermometer is 100% accurate, but unless there is some voodoo science going on here, there certainly isn't a 5 - 10 degree difference in temperature between the wort at the edge of the carboy vs. that at the center.

 

[david_42]

This idea was disproven in a series of experiments 3-4 years ago. Comparisons of outside temperature to the inside temperature was demonstrated to be equal within the limits of the thermometers. Even for a glass carboy. (Limited to 5 & 10 gallon fermenters)

There is a difference between wort temperature and ambient air temperature.

 

[dwarven_stout]

Since I don't have an actively fermenting wort right now to drop two thermometers in, I'll just post some thoughts.

I'm not sure that your assumption of a well-mixed system is valid.

A carboy is wide and short; I expect that natural (laminar flow) convection is dominant for fermenting wort. The only source of turbulent flow should be from rising CO2 bubbles and transported mass (yeast, trub). Warmer wort will lose some heat to the top(air) and sides of the carboy and sink towards the bottom, regaining heat from fermentation and rising (warm) mass transported by gas bubbles.

Assuming the heat of fermentation is produced roughly equally throughout the wort (valid, from my recalled reading), we should expect the center of the wort to be hotter than the sides (that are nominally in thermal contact with the carboy). I'm not sure if this would be necessarily true for a 5 gallon carboy, but I bet it is for a larger system. I never studied heat transfer, though.. I might be off-base. Maybe I'll drop thermometers into my next batch.

Edit: Looks like I got scooped while I was reading other tabs. Happen to have a link to those experiments, David?

 

[SpanishCastleAle]

FWIW, I 3/4-submerge my carboy in water during ale fermentation. I measure the water bath temp and the temp of the wort at the center of the carboy (SS thermowell sticking through carboy cap). The temp at the center of the carboy is within 1 degree or so of the water bath temp even at the height of fermentation. This is totally different from air (much better cooling) but the point is that the temp is relatively constant throughout. If anything the gradient should be 'vertical'.

I just bought a new brew kettle with a port for a SS thermometer. Now that is way off...the thing reads the pot temp as much as it does the contents. Significant temp swings of the contents showed no temp change on the outer thermometer.

 

[Bobby_M]

Well, there are actually two different things here.

1. Is the ambient air temp different than the actively fermenting wort temp? Absolutely. I've had a barleywine reading 10F+ over ambient.

2. Is the core of wort in the fermenter a different temp than say the outer 1" circumference? If there is any gradient it would be very small if only the ambient temp is way different than the average wort temp. It would be very small due to the currents you mentioned.

A paste-on LCD thermo probably shows an average between wort temp and ambient on a glass carboy or bucket, but it's probably much closer to wort temp on a stainless or plastic better bottle. I always assume my wort is about 1-2F warmer than it read on my better bottle because that's what I measured the one time I drew a sample for gravity testing.

 

[brewmonk]

All the theory and assumptions aside, I don't need absolute temperature, just relative.
(I've never considered temp. gradients, and currents, and stuuf that makes my eyes spin.)

Whatever temp it is, when it drops 4-5degF, I start checking.

 

[BuzzCraft]

3) Fermentation is an active process (i.e. fermenting wort is not stagnant, it moves)
QUOTE]

The reason the fermenting wort moves is BECAUSE OF temperature gradients, no? The wort at the edges of the fermenters cools and drops, setting up the currents we witness.

But is there a meaningful gradient? I doubt it.

 

[Houblon]

I used this for awhile and yes the center can be 8-10f over ambient but the wort is in constant motion so I doubt the outer 1" circumference is much cooler than the center. But I will always assume the stick-on thermo is ready 5f min~ cooler than the wort due to the plastic`s ability to insulate.

 

[SpanishCastleAle]

The reason the fermenting wort moves is BECAUSE OF temperature gradients, no? The wort at the edges of the fermenters cools and drops, setting up the currents we witness.

It's more due to CO2 bubbles rising than anything.

 

[SPThirtyThree]

I've read these forums for a couple years, and finally I get to contribute something...

I have actually been debating this topic with a buddy recently, so I did some calculations. With the following assumptions:

5-gallon brew
SG 1.065
75% attenuation
20% heat loss from expelled CO2

Fermentation converts one molecule of glucose into two molecules of ethanol, two of CO2, two ATP, and some heat. The heat is 219 kJ/mol glucose. So for this batch, 2.39 kg of glucose gets converted to delicious ethanol (assuming glucose is worth 386.5 pts/kg/L - not 100% sure of that).

If you do the math, you'll see that the beer will generate 2900 kJ during fermentation. Interestingly, this would raise the temperature by about 65° F if it happened spontaneously. However, assuming the max fermentation rate is .22kg glucose/hL/h (Fricker, 1978), then the fermentation for this batch generates 14W heat, about 3 watts of which is carried out of the carboy with the CO2. This would be worst case - that is, I've assumed the wort will instantly start fermenting at the maximum rate, and will stay at that rate until it is finished, rather than actually ramping up and then slowly tapering off. For this batch, that theoretical time period at max fermentation would be 57 hours.

I'm still waiting for a Flotherm license to open up at here at work to verify the following thermal assumptions, but intuitively, the fluid will be at a constant temperature, due to the reasons listed above (vigorous movement and high thermal conductivity), so the only possible temp gradient would be in the glass.

If the only heat is lost through the walls of the carboy, which I assume are around .5" thick, then the gradient in the glass wall is only 0.10°F, because ΔT = Q*x/k/A, where Q is the heat generation rate (set by Fricker's calculation), x is the glass thickness, k is the thermal conductivity of glass (1.1 W/m/K), and A is the area through which the heat must pass.

So based on my calculations/assumptions: the wort is effectively all the same temperature, and the outside of the glass is going to be about a tenth of a degree cooler than the wort. This seems reasonable, eleven watts really isn't much heat, so the overall gradient from the glass surface to the middle of the wort is probably within the error of the thermometer.

The significance is that the wort's temperature is governed by ambient, rather than internal heat generation.

I have the spreadsheet if anyone is interested. Having an engineering mind ruins pretty much everything...

 

[maida7]

But I will always assume the stick-on thermo is ready 5f min~ cooler than the wort due to the plastic`s ability to insulate.

Please explain... Are you saying that the stick on thermometer is typically off by 5 degrees F? I just don't see how that is possible.

I think the stick on thermometer is usually spot on.

Except in extreme cases where the ambient air temp outside the fermenter is drastically different the the wort temp. For example when you put your 80F wort into a 35F refrigerator to cool it down fast before pitching the yeast. Then the stick on thermometer will be way off.